This invention relates to a magnetron, more particularly to a magnetron of the type wherein permanent magnets are contained in a sealed evacuated vessel.
This type of magnetron is disclosed in Japanese patent laid open specification No. 114162/1975 and U.S. Pat. No. 4,042,851 for example. According to the construction of the magnetron of this type since it is possible to dispose the permanent magnets close to the interaction space defined between the cathode electrode and anode vanes it is possible to decrease the leakage flux in the magnetic path. As a consequence, it is possible to efficiently utilize the magnetic flux generated by the permanent magnets and hence to decrease the size thereof and the magnetron.
With this type of magnetron, however, as a yoke disposed adjacent an antenna conductor and adapted to support one surface of a magnet, a pole piece secured to the other surface of the magnet, supporting members of the magnet and pole piece, and an antenna support are utilized as a microwave transmission circuit for deriving the output from the magnetron, unless the material, dimensions and surface treatments of these component parts are controlled strictly, the following problems occur.
More particularly, unless the component parts are maintained in an intimate electrical contact, electric field concentrates at poorly contacted portions to form electric spark. This causes power loss, decreases output efficiency and emits gas due to heat. This degrades the electron emission characteristic of the cathode filament thereby causing such abnormal phenomena as moding and degrading the stability and reliability of the magnetron.
Magnets utilized in these magnetrons are usually made of cast Alnico (trade name) or a samarium-cobalt alloy but the magnetization characteristics of these materials deteriorate when the temperature exceeds 500.degree. C. or 600.degree. C.
For the purpose of obviating these problems it has been the practice to finish at high accuracies the permanent magnets and component parts connected thereto which increases the cost of the magnetron. However, since the pole pieces are generally formed by press working soft steel sheets, the pole pieces tend to flex after working and when such flex occurs, electric field concentrates at a gap formed by the flexed pole piece. When foreign matter may happen to be interposed between the permanent magnet and a member connected thereto, field concentration also occurs.